(1) Field of the Invention
The present invention relates to a turning control apparatus for a vehicle.
(2) Description of Related Art
Techniques have been developed for improving safety of vehicles by achieving vehicle-stability during turning maneuvers. For example, JP H9-86378 Laid Open Publication discloses a feedback control, based on the yaw-rate of the vehicle, for both the torque that varies between the left and right wheels and also the different brake pressures of individual wheels.
Further, JP 2853474 Patent Publication discloses another feedback control for the Electrical Controlled Limited Slip Differential Unit of the center differential that varies the speeds between the front and rear wheels.
However, it is difficult to eliminate over-steer of vehicles during turning maneuvers by such techniques as controlling the left-right-wheel driving torques in JP H9-86378.
When a vehicle is turning, a conventional controller, which uses such techniques as those disclosed in JP H9-86378, increases the driving torque of the inside wheel, which is one of either the right or left wheels which is nearest to a center of an axis of turning.
When a vehicle is turning, the inside wheel load is decreased and the outer-side wheel load is increased, relatively. The inside wheel's grip on the road surface, that is the traction of the inside wheel, is decreased accordingly.
In this situation, the inside wheel skids easily if the inside wheel's driving torque is increased, therefore it is difficult to generate sufficient yaw of the vehicle to eliminate over-steering. Further, skidding of the inside wheel is more pronounced during acceleration of the vehicle.
During deceleration of the vehicle, the inside wheel traction is decreased and the front wheel load is increased. Accordingly, the rear wheel's load is decreased and the traction of the rear wheels is decreased, relatively.
As explained above, it is difficult to eliminate not only over-steering but also under-steering during turning maneuvers by such controlling that varies the driving torques between the left and right wheels, consequently.
Although, it is possible to accept such a technique that permits increasing the differential speed between the front and rear wheels to enhance the turning ability of a four-wheel drive vehicle in the under-steer condition, the acceleration ability of the vehicle is lost, because the overall traction of the four-wheel drive vehicle is decreased.
Assume that the rear wheel slips when the center differential does not limit the speed difference between the front and rear wheels, the rear wheel rotates faster. In this situation, the driving torque, which should be transmitted to the front wheel, is almost all transmitted to the rear wheels and consequently the acceleration is prevented.